In an airless, piston-type paint spray gun, a piston pumps the paint from a supply line and into a paint holding chamber. The gun has a trigger which, when depressed, opens a valve to cause the pressurized paint in the chamber to be sprayed out of a nozzle in the direction of a surface to be coated.
For ideal spraying conditions, the pressure of the paint in the chamber should be in the range of about 1800-2100 p.s.i. This pressure range is maintained by controlling the operation of a motor which drives the piston pump. More particularly, a diaphragm pressure switch senses the fluid pressure of the paint in the chamber, and in response, generates a signal to the motor to either start or stop the piston. According to a typical sequence of operation, the chamber is initially empty, and therefore the fluid pressure is zero. A "pump" signal to the motor causes the piston pump to begin, thereby commencing priming of the pump. During priming, an electrical contact of the diaphragm pressure switch remains in a normally-closed position. The contact is in series with a conductive lead which conveys the pump signal to the motor. At a predetermined pressure, i.e. an upper limit for paint pressure in the chamber, a diaphragm in the switch deflects to displace a spring-biased piston and open a piston, thereby opening the normally-closed electrical contact. Opening of the electrical contact terminates the pump signal to the motor and de-energizes the pump.
Thereafter, when the trigger is depressed, pressurized paint from the chamber will be expelled through the nozzle. As a result, the fluid pressure of the paint in the chamber decreases. At a predetermined lower limit for paint pressure in the chamber, i.e. about 1700 p.s.i., the diaphragm begins to move toward its original position, and the piston moves with the diaphragm, due to the spring. Eventually, the diaphragm and piston move a sufficient distance to close the contact and deactivate the switch, thus restarting the motor which drives the pump. The pump operates until the pressure diaphragm switch again senses that the upper pressure limit has been reached, whereupon the contact again opens to disconnect the pump. In this manner, the switch provides continuous, cyclical control of the motor which drives the pump.
The pressure diaphragm switch plays an important role in maintaining sufficient spraying pressure for an airless, piston-type spray gun. In one common diaphragm pressure switch, opening of the electrical contact is caused by mechanical contact with one end of a switch piston. An opposite end of the piston contacts the rear surface of a deflectable diaphragm. The front surface of the diaphragm contacts the fluid to be sensed. The diaphragm has a diameter which is greater than that of the piston head. The diaphragm spans across a recess in the forward end of a hollow, cylindrical switch body. In use, the recess is in fluid communication with the paint chamber, and the diaphragm deflects according to the fluid pressure of the paint in the recess. The switch body houses the piston and a spring. The head of the piston extends through an opening in a transverse wall at a forward end of the switch body. The spring biases the piston head into contact with the back surface of the diaphragm.
For an airless, piston-type paint spray gun, it is desirable to provide a diaphragm pressure switch which outlasts the useful life of the spray gun, thereby eliminating the need for replacement or servicing of parts. Unfortunately, the structural and environmental requirements for the diaphragm of a diaphragm pressure switch have proved that this objective is more easily stated than attained.
More particularly, the diaphragm must deflect with changes in fluid pressure in a manner which is predictable and repeatable, within the necessary pressure ranges, for its entire useful life. Because the deflection of the diaphragm must displace a piston, the diaphragm must be of a relatively sturdy material. Moreover, for optimum accuracy and repeatability in fluid pressure sensing, it is also desirable to locate the diaphragm in direct fluid contact with the paint in the chamber. Unfortunately, because of the chemically-corrosive nature of many types of paint, a diaphragm for use in an airless, piston-type paint spray gun must be made of a material which is chemically inert, or not susceptible to chemical corrosion from the paint.
These diverse requirements limit the number of materials which are suitable for use in a pressure diaphragm switch. Rubber is a suitable diaphragm material from the standpoint of both deflectability and relatively low cost. However, rubber is particularly susceptible to chemical corrosion from paint solvents. Rigid plastic has also been used in the past as a diaphragm material. Unfortunately, when contacting sharp metal edges such as the side edges of a switch piston, these rigid plastics tend to extrude around the edges of the piston.
Prior attempts to combine two diverse materials in a single diaphragm have also failed. One attempt involved a diaphragm which included a rigid plastic layer in direct contact with the paint and a backing layer of rubber located between the switch body and the plastic layer. Unfortunately, this two-component diaphragm needed repair after only six hours of use. By comparison, the normal useful life of an airless, piston-type paint spray gun is about 75 hours.
One problem with this particular two-component diaphragm related directly to the relatively high pressures used in this type of paint spray gun. More specifically, at above 1000 p.s.i., the rubber backing layer became squeezed to a paper thin width between the plastic layer and the piston, causing the excess rubber to move outwardly from therebetween and obstruct or inhibit normal movement of the piston, particularly in returning to its original, undeflected position, and thereby reducing the sensitivity of the switch. In some cases, the diaphragm and piston did not return to their undeflected positions until the fluid pressure had lowered to about 1000 p.s.i., resulting in a hysterisis range for the switch of about 800 to 1000 p.s.i. While some hysterisis for a diaphragm pressure switch is acceptable, this amount of hysterisis for the diaphragm pressure switch used in a piston-type paint spray gun is too great. Due to this hysterisis, the electrical contact remains open too long, and the pump is not energized quickly enough to sufficiently pressurize the paint in the chamber for adequate spraying when spraying is resumed.
It is an objective of the invention to extend the useful life of a diaphragm for a pressure diaphragm switch beyond the useful life of an airless, piston-type paint spray gun in which it is used.
It is another objective of the invention to reduce the fluid pressure hysterisis of a diaphragm pressure switch used in an airless, piston-type paint spray gun.
It is still another objective of the invention to minimize the effects of chemical corrosion, the cost and the pressure susceptibility of a diaphragm used in a pressure diaphragm switch.
The objectives of this invention are met by a diaphragm for a diaphragm pressure switch which includes a deflectable, chemically-inert primary member which is disc-shaped and made of rigid plastic and a resilient, ring-shaped, secondary member for supporting the primary member. The resilient, washer-shaped secondary member supports the periphery of the rear surface of the primary member around the outside of the piston head.
The primary member is rigid plastic, to withstand chemical corrosion from direct contact with paint solvents and also to shield the rubber secondary member from contact with the paint. At the same time, the rubber secondary diaphragm provides resilient support for the deflectable primary member, thereby eliminating stress on the primary member caused by the sharp edges of the piston and reducing the susceptibility to undesired extrusion of the primary member around the sharp edges of the piston under high pressure conditions.
Under normal operating conditions, i.e. paint chamber pressures ranging from 0-2700 p.s.i., the dual component diaphragm structure of this invention extends the useful life of a diaphragm pressure switch beyond the useful life of an airless, piston-type paint spray gun in which it is used. For a useful life which exceeds the normal seventy-five-hour life span of a paint spray gun, a pressure diaphragm switch equipped with this dual component diaphragm is capable of accurately sensing paint pressure and effectively controlling the operation of the piston pump. During its useful life, the hysterisis of this dual component diaphragm remains relatively constant, in the range of about 300-400 p.s.i. Thus, with this diaphragm, the switch may be set to accurately and repeatably turn off the piston pump when pressure in the chamber reaches 2000-2400 p.s.i. and to turn on the piston pump again when the paint pressure falls to about 1600-1700 p.s.i.
According to a preferred embodiment, this dual component diaphragm is located within a recess at a forward end of a hollow, cylindrical switch body. The switch body connects to the pump housing to place the recess in fluid communication with the paint chamber. Together, the recess and the chamber from a fluid sensing zone. The dual component diaphragm defines one end of the sensing zone. The diaphragm deflects with changes in the fluid pressure of the paint in the chamber. Because a central region of the primary member contacts the head of a linearly displaceable, spring-biased piston, the diaphragm deflections displace the piston. This piston displacement opens a normally-closed electrical contact in the diaphragm pressure switch, thereby de-energizing the motor which drives the piston pump.
With this switch, the distance between the piston and the contact may be varied to change the desired upper pressure limit for de-energizing the motor. Preferably, during painting, the paint pressure in the chamber should be maintained at about 1800-2100 p.s.i.
These and other features of the invention will be more readily understood in view of the following detailed description and the drawing.